The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of pulverulent coating compositions based on polyamides having at least ten aliphatically bound carbon atoms per carbonamide group by the precipitation method. Copolyamides or a mixture of homopolyamides and copolyamides containing at least 70% of the stated componenets are also useful.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Patents Nos. 3,476,711; 3,900,607; 3,927,141; 3,966,838; 4,143,025; 4,195,162; 4,273,919; and 4,334,056; British Patents Nos. 535,138; 688,771; and 1,392,949; and the Kirk-Othmer "Encyclopedia of Chemical Technology:, 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92 - polylauryllactam (nylon-12), and polyundecanamide (nylon-ll), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein by reference.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822.
It is known to use polyamide based pulverulent coating compositions in the preparation of varnish type coatings on metals. The coating method is performed by the melt film procedure, namely the fluidized bed method, the flame spray process or the electrostatic coating method. The polyamide powders are obtained by precipitating the polyamide from solutions as disclosed in British Patent No. 688,771, or by grinding the polyamide granulate, preferably at low temperatures and in an inert gas atmosphere as disclosed in U.S. Pat. No. 4,273,919.
British Patent No. 688,771 discloses the precipitation of polyamide powders from ethanol by cooling the hot polyamide solution by external means or by merely letting it stand to cool. It is only by forced cooling or by letting the hot polyamide solution stand to cool that powders with a high proportion of fine grains are obtained, which lead to dust generation in the fluidized bed. Lastly such a procedure causes undesired agglomerations which subsequently require abrasion or grinding to be put into the required powder size. Moreover such methods result in nonreproducible batches which vary in particle size, bulk density, and molecular weight.
It is further known to manufacture polyamide powders by grinding polyamides of low molecular weights and by obtaining the desired viscosity in the powders so made by heating them to temperatures below the melting point as disclosed in U.S. Pat. No. 3,471,711.
Polylauryllactam powders also are prepared by this known method and are put to use again by known procedures as disclosed in Chem. Ind. November 1968, pages 783 to 791, and Modern Plastics, February 1966, pages 153 to 156. Because the polylauryllactam powders do not always meet the conditions of high elasticity, good edge coating, smooth surface, resistance to alkaline aqueous solutions, and frequently emit thick smoke during processing, among other drawbacks, the most diverse improvements have already been disclosed, such as polylauryllactam powders containing plasticizers, as disclosed in U.S. Pat. No. 3,900,607; or powders made of a mixture of copolyamides containing homopolylauryllactam and lauryllactam, as disclosed in British Patent No. 1,392,949, or those containing polyamides having N-alkoxymethyl groups in addition to acidically reacting catalysts, as disclosed in U.S. Pat. No. 3,966,838, or those with mixtures of polyamides having 8 to 11 aliphatically bound carbon atoms per carbonamide group, aminoplasts bearing alkoxyalkyl groups and acidically reacting catalysts, as disclosed in U.S. Pat. No. 3,927,141. These powders have good properties in specific instances but fall short of meeting all the requirements.
An improved method is described in U.S. Pat. Nos. 4,143,025 and 4,195,162. This procedure, however, still falls short because a grinding method is employed to prepare pigment-free powders and the precipitation method must be used to make pigmented powders. Lastly, this method presumes the use of polylauryllactam granulates which were prepared exclusively by hydrolytic polymerization in the presence of specified amounts of phosphoric acid.
U.S. Pat. No. 4,334,056 discloses a further improved method for preparing coating powders by the precipitation procedure comprising a method of producing polyamide powder from polyamide having a relative viscosity of 1.4 to 1.8 as measured in 0.5% meta-cresol solution at 25.degree. C. and at least 70% of said polyamide consisting of polyamide with at least ten aliphatically bound carbon atoms per carbonamide group, comprising:
(a) dissolving said polyamide in at least twice the amount by weight of ethanol in a closed vessel at a temperature between about 130.degree. to 150.degree. C. to form a solution of said polyamide;
(b) cooling said solution to a precipitation temperature between about 100.degree. to 125.degree. C. and ceasing said cooling at said precipitation temperature;
(c) precipitating said polyamide powder from said cooled solution of (b) polytropically with agitation and under an inert gas atmosphere; and
(d) separating said precipiatated polyamide powders of (c) from said ethanol.
Coating powders prepared by the precipitation method have a somewhat more advantageous distribution of grain size than those made by grinding, and furthermore the granins are also somewhat smoother at the edges. Nevertheless they must be graded by screening or sifting to remove the interfering portions of small or large particles.